Together with drastic increase of the communication demand in recent years, development of a wavelength division multiplexing communication system (WDM communication system) which multiplexes a plurality of signal lights having wavelengths different from each other so that high-capacity transmission may be implemented with one optical fiber is advancing.
In such a wavelength division multiplexing communication system as just described, in order to implement a flexible and an-advanced communication system, a wavelength tunable laser capable of selecting a required wavelength over a wide wavelength range at a high speed is needed.
For example, as a wavelength tunable laser capable of changing over the wavelength at a high speed, a DBR (Distributed Bragg Reflector) laser, a TTG-DFB (Tunable Twin Guide-Distributed Feedback) laser, a TDA-DFB (Tunable Distributed Amplification-Distributed Feedback) laser and so forth have been proposed.
In such wavelength tunable lasers as mentioned above, since variation of an equivalent refractive index of an optical waveguide where current is injected to the optical waveguide is utilized to vary the oscillation wavelength, changeover of the oscillation wavelength can be carried out in the order of several nm.
Particularly, the TDA-DFB laser can be configured such that a mode hopping does not occur and can carry out wavelength control at a high speed through simple control. Further, since a device for the TDA-DFB laser can be produced readily, the TDA-DFB laser is prospective as a wavelength tunable laser for communication.
However, a wavelength tunable laser such as a TDA-DFB laser which can utilize refractive index variation by current injection to an optical waveguide and prevent occurrence of a mode hopping in such a manner as described above cannot fully cover the C-Band (1,525 to 1,565 nm) which is important in the WDM communication system because generally the wavelength variation width thereof is approximately several nm.
Therefore, an array integration type wavelength tunable laser has been proposed wherein a plurality of such wavelength tunable lasers as described above are monolithically integrated in the form of an array in one device so that the wavelength band described above is covered with one device.
For example, since the tuning range (the wavelength variation width) of the TDA-DFB laser is approximately 5 nm, by integrating eight TDA-DFB lasers configured such that the oscillation wavelength ranges thereof are different from each other by 5 nm in one device, a wavelength tunable laser which can oscillate over the overall C-Band can be implemented.